As illustrated in FIG. 1 of the enclosed drawings, the known injection machines of liquid metal, generally aluminum, are provided with a base portion 1, in which a fixed mold portion 2 is affixed, configured to cooperate with a movable mold portion 3, which is displaced between an open position (not illustrated) and a closed position, illustrated in FIG. 1, in which it defines a molding cavity 4 with the fixed mold portion 2 in a well known prior art constructive arrangement.
The base portion 1 of the injection machine carries a tubular body 10 transversally extended along the fixed mold portion 2 and which is designed to internally define a cylindrical and elongated injection chamber CI. Said tubular body 10 is generally horizontally disposed and presents a closed mounting end 11, through which is tightly slidably mounted a rod 5a of a piston 5 axially displaced in the interior of the injection chamber CI, between a retracted position, as illustrated in FIG. 1, in which said piston 5 is retracted close to the mounting end 11 of the tubular body 10, and an injection position, which is reached after said piston 5 is displaced throughout the injection chamber CI, close to an outlet end 12 of the tubular body 10, opposite to the mounting end 11 and opened to the interior of the molding cavity 4, compressing a charge of molten metal MF, already fed to the interior of the injection chamber CI, to the interior of the molding cavity 4.
The charge of molten metal MF is fed into the injection chamber CI, generally through a shell 6 (or chute coming from a dosing furnace) of known construction, and through a supply window 13, radially provided in the tubular body 10, close to the mounting end 11 of the latter, but in a position axially ahead of the piston 5, when in its retracted inoperative position.
In this type of construction, when the molten metal MF is poured to the inside of the injection chamber CI, through the supply window 13, there occurs heat exchange from the molten metal MF to the tubular body 10 of the injection chamber CI. Depending on the metal volume to be supplied and on the time for pouring the charge required for each injection cycle, said heat exchange can be of such magnitude that the usual temperature control systems are insufficient to maintain the wall temperature of the tubular body 10, in the region which receives the pouring of the molten metal MF, within acceptable values which do not alter the characteristics of the conventional hot-work steel or metallic alloy, in the formation of the tubular body 10.
In the case of aluminum, for example, the temperature of the molten metal MF is of about 680 degrees Centigrade and, depending on the charge and on the time of each pouring operation, the adjacent wall region of the tubular body 10 can be excessively heated, reaching temperatures capable of altering the microstructure of the conventional hot-work steel, provoking surface stresses and generating cracks which tend to progressively increase, leading to an early erosion of the inner region of the tubular body 10, against which the charge of molten metal MF is poured.
As is known by those skilled in the art, the erosion of the inner region of the injection chamber CI allows the molten metal to be housed in this eroded region, impairing the liquid metal injection process, as well as damaging the chamber and the piston in a degree capable of causing the binding of the piston and the consequent destruction of the piston-chamber assembly.
The inconvenience related to the early erosion mentioned above is even more harmful to the machine and to the efficiency of its operation, when the temperature control system of the tubular body is designed only for cooling the region subjected to the heating provoked by the pouring of the molten metal MF.
The limitations regarding the wall thickness of the tubular body 10 impair the provision of linings and also the adequate dimensioning of the temperature control system, making it incapable of preventing the temperature from quickly and undesirably rising in each pouring and injection cycle of molten metal MF, mainly in the cases of large pouring volumes, which occurs in injection chambers having a diameter of 100 mm or more. In these cases, the undue heating of the tubular body 10 and the early erosion are unavoidable.
In the systems in which there is no effective control of the temperature of the tubular body 10, but only the cooling to reduce the heating degree of the injection chamber, generally there occurs another problem resulting from the undue cooling of the molten metal remaining in the interior of the chamber, between two injection cycles. This metal tends to cool and modify the microstructure of the piece to be produced in the mold, making said microstructure inappropriate and also producing a “cold junction”.